As a technique for a bit replacing device which replaces the abraded roller bit for another roller bit from the workspace formed in the cutter head during excavation, for example, Patent Literature 1 and Patent Literature 2 have been proposed. The bit replacing devices each include a rotor having an opening and disposed on the front surface of a cutter spoke. A roller bit is disposed in the opening of the rotor. When replacing the roller bit, the rotor is turned by 90° or 180° to cause the opening of the rotor to face an opening for replacement provided on the side surface side or rear surface side of the opening of the rotor. Thus, the roller bit is removed out from the opening of the rotor into the workspace via the opening for replacement.